An electrochemical system is a system that either derives electrical energy from chemical reactions, or facilitates chemical reactions through the introduction of electrical energy. An electrochemical system generally includes a cathode, an anode, and an electrolyte, and is typically complex with multiple heterogeneous subsystems, multiple scales from nanometers to meters. Examples of these systems include fuel cells, batteries, and electroplating systems. On-line characterization of batteries or fuel cells in vehicles is difficult, due to very rough noisy environments.
On-line characterization of such electrochemical systems is desirable in many applications, which include real-time evaluation of in-flight batteries on a satellite or aviation vehicle, and dynamic diagnostics of traction batteries for electric and hybrid-electric vehicles. In many battery-powered systems, the efficiency of batteries can be greatly enhanced by intelligent management of the electrochemical energy storage system. Management is only possible with proper diagnosis of the battery states.
Although there may be many kinds of characterization models for an electrochemical system, equivalent circuit models are most appropriate in many applications where stringent real-time requirements and limiting computing powers need to be considered. An algorithm for a circuit model is relatively simple, meaning that simulation time is short and the computation cost is relatively low. A circuit model is an empirical model that describes the electrochemical system with a resistor-capacitor (or resistor-inductor-capacitor) circuit.
In a suitable circuit model, major effects of thermodynamic and kinetic processes in the electrochemical system can be represented by circuit elements. For example, the electrode potential between the cathode and the anode of a system can be represented with a voltage source, the charge-transfer processes can be represented with charge-transfer resistances, the double-layer adsorption can be represented with capacitances, and mass-transfer or diffusion effects can be represented with resistances such as Warburg resistances. Therefore a circuit model is extremely useful for many on-line diagnostics of the real-time states of an electrochemical system.
Conventionally, a thermocouple is used to measure the temperature of a device or structure, but very often, it is very difficult and troublesome to embed the thermocouples into a system, such as the case of an energy storage device (e.g., lithium-ion batteries). Infrared imaging is another technique for thermal imaging. However, this method not only requires an IR camera, but it is also very difficult to quantify the internal temperature of the battery. The current technologies very often only measure the outside or surface temperatures, which do not reflect what is going on inside a device. It is known that for many batteries such as a secondary Li-ion battery, impedances are sensitive to their own inner temperatures (see, for example, US2012/0099618).
Monitoring the internal temperature of a battery is critical for battery testing/evaluations, efficient battery management, and safety. Conventional methods of monitoring the ambient battery temperature or the temperature of a battery's external casing are a poor indication of the internal temperature of the cell. Because the thermal conductivities of the active material layers are less than those of the metals, depending on the heating rate, the internal temperature can vary significantly from the external temperature. This is especially problematic when localized internal heating occurs; poor thermal conductivity of the battery layers can produce large temperature gradients between the internal and external components of the battery.
A system that is capable of monitoring the internal cell temperature could provide safer, more efficient management of batteries and other electrochemical devices. Improvements are desired for more-accurate estimates of internal temperatures in these electrochemical devices.